fixup! sys/net/nanocoap: fix coap_build_reply_header()

sys/net/application_layer/nanocoap/nanocoap.c

@@ -592,20 +592,21 @@ ssize_t coap_build_reply_header(coap_pkt_t *pkt, unsigned code,
         }
     }
 
+    /* we need to update the header length with the actual one, as we may have
+     * used less bytes for the extended token length fields as our worst case
+     * assumption */
+    hdr_len  = bufpos - (uint8_t *)buf;
+
     if (payload) {
         if (ct >= 0) {
             bufpos += coap_put_option_ct(bufpos, 0, ct);
         }
         *bufpos++ = COAP_PAYLOAD_MARKER;
+        hdr_len  = bufpos - (uint8_t *)buf;
         *payload = bufpos;
         *payload_len_max = len - hdr_len;
     }
 
-    /* we need to update the header length with the actual one, as we may have
-     * used less bytes for the extended token length fields as our worst case
-     * assumption */
-    hdr_len  = bufpos - (uint8_t *)buf;
-
     /* with the nanoCoAP API we can't detect the overflow before it happens */
     assert(hdr_len <= len);